Acetylcholine released from cholinergic neurons in the peripheral and central nervous systems affects many different biological processes through interaction with two major classes of acetylcholine receptors—the nicotinic and the muscarinic acetylcholine receptors. Muscarinic acetylcholine receptors (mAChRs) belong to the superfamily of G-protein coupled receptors that have seven transmembrane domains. There are five subtypes of mAChRs, termed M1-M5, and each is the product of a distinct gene. Each of these five subtypes displays unique pharmacological properties. Muscarinic acetylcholine receptors are widely distributed in vertebrate organs where they mediate many vital functions. Muscarinic receptors can mediate both inhibitory and excitatory actions. For example, in smooth muscle located in the airways, M3 mAChRs mediate contractile responses. For a review, see Caufield, Pharmac. Ther. 58, 319 (1993).
In the lung, mAChRs have been localized to smooth muscle in the trachea and bronchi, the submucosal glands, and the parasympathetic ganglia. Muscarinic receptor density is greatest in parasympathetic ganglia and then decreases in density from the submucosal glands to tracheal and then bronchial smooth muscle. Muscarinic receptors are nearly absent from the alveoli. For a review of mAChR expression and function in the lungs, see Fryer and Jacoby, Am. J. Respir. Crit. Care Med. 158, 154 (1998).
Three subtypes of mAChRs have been identified as important in the lungs, M1, M2 and M3 mAChRs. The M3 mAChRs, located on airway smooth muscle, mediate muscle contraction. Stimulation of M3 mAChRs activates the enzyme phospholipase C via binding of the stimulatory G protein Gq/11 (Gs), leading to liberation of phosphatidyl inositol-4,5-bisphosphate, resulting in phosphorylation of contractile proteins. M3 mAChRs are also found on pulmonary submucosal glands. Stimulation of this population of M3 mAChRs results in mucus secretion.
M2 mAChRs make up approximately 50-80% of the cholinergic receptor population on airway smooth muscles. Although the precise function is still unknown, they inhibit catecholaminergic relaxation of airway smooth muscle via inhibition of cAMP generation. Neuronal M2 mAChRs are located on postganglionic parasympathetic nerves. Under normal physiologic conditions, neuronal M2 mAChRs provide tight control of acetylcholine release from parasympathetic nerves. Inhibitory M2 mAChRs have also been demonstrated on sympathetic nerves in the lungs of some species. These receptors inhibit release of noradrenaline, thus decreasing sympathetic input to the lungs.
M1 mAChRs are found in the pulmonary parasympathetic ganglia where they function to enhance neurotransmission. These receptors have also been localized to the peripheral lung parenchyma, however their function in the parenchyma is unknown.
Muscarinic acetylcholine receptor dysfunction in the lung has been noted in a variety of different pathophysiological states. In particular, in asthma and chronic obstructive pulmonary disease (COPD), inflammatory conditions lead to loss of inhibitory M2 muscarinic acetylcholine autoreceptor function on parasympathetic nerves supplying the pulmonary smooth muscle, causing increased acetylcholine release following vagal nerve stimulation (Fryer et al., Life Sci. 64, 449 (1999)). This mAChR dysfunction results in airway hyperreactivity and hyperresponsiveness mediated by increased stimulation of M3 mAChRs. Thus the identification of potent mAChR antagonists would be useful as therapeutics in these mAChR-mediated disease states.
COPD is an imprecise term that encompasses a variety of progressive health problems including chronic bronchitis and emphysema, and it is a major cause of mortality and morbidity in the world. Smoking is the major risk factor for the development of COPD; nearly 50 million people in the U.S. alone smoke cigarettes, and an estimated 3,000 people take up the habit daily. As a result, COPD is expected to rank among the top five diseases as a world-wide health burden by the year 2020. Inhaled anticholinergic therapy is currently considered the “gold standard” as first line therapy for COPD (Pauwels et al., Am. J. Respir. Crit. Care Med. 163, 1256 (2001)).
Despite the large body of evidence supporting the use of anticholinergic therapy for the treatment of airway hyperreactive diseases such as COPD, relatively few anticholinergic compounds are available for use in the clinic for pulmonary indications. More specifically, in the United States, ipratropium (Atrovent; also as Combivent in combination with albuterol) and tiotropium (Spiriva) are currently the only inhaled anticholinergics marketed for the treatment of hyperreactive airway diseases. While the latter is a potent and long-acting anti-muscarinic agent, it is not available as a combination with other pharmacological agents such as albuterol. This appears to be due to the lack of sufficient chemical stability of tiotropium in the presence of certain additional agents.
Thus, there remains a need for novel anticholinergic agents, i.e., agents that inhibit the binding of acetylcholine to its receptors, which can be co-formulated with other pharmaceuticals and which can be administered conveniently, such as once a day, for the treatment of hyperreactive airway diseases such as asthma and COPD.
Since mAChRs are widely distributed throughout the body, the ability to apply anticholinergic agents locally and/or topically to the respiratory tract is particularly advantageous, as it would allow for lower doses of the drug to be utilized. Furthermore, the ability to design topically active drugs that have long duration of action, and in particular, are retained either at the receptor or by the lung, would avoid unwanted side effects that may be seen with systemic anticholinergic exposure. However, other muscarinic acetylcholine receptor-mediated diseases respond to systemic administration. Thus, medications useful for respiratory disorders can be administered systemically when appropriate for treatment of the respiratory disorder, or when appropriate for treatment of a non-respiratory disorder.